1. Field of the Invention
The present invention relates to a process for the preparation of 3,5,5-trimethyl-cyclohex-2-ene-1,4-dione from .beta.-isophorone (3,5,5-trimethyl-cyclohex-3-en-1-one).
This diketone is useful in that it can be aromatized to give trimethylhydroquinone, which is known as an important intermediate in the synthesis of vitamin E. The conversion of 3,5,5-trimethyl-cyclohex-2-ene-1,4-dione to trimethylhydroquinone can be carried out, according to the process described in German Patent application No. 2,149,159, by the action of an acylating agent in the presence of a protonic acid of pKa &lt; 3, or of a Lewis acid, followed by hydrolysis of the resulting diester.
2. Description of the Prior Art
Various methods for the preparation of 3,5,5- trimethyl-cyclohex-2-ene-1,4-dione have been proposed, which start either from .alpha.-isophorone (3,5,5-trimethyl-cyclohex-2- en-1-one) or from .beta.-isophorone, which is easily obtained by isomerization of .alpha.-isophorone, by subjecting the latter to a slow distillation in the presence of a sulphonic acid (see French Pat. No. 1,446,246). Thus, according to French Pat. No. 2,213,264, the oxidation of .alpha.-isophorone can be carried out by means of an alkali metal chromate or bichromate or chromium trioxide in a mixture of acetic acid and acetic anhydride. This process requires the use of a large excess of oxidizing agent relative to the starting ketone in order to obtain yields of about 50% based upon the .alpha.-isophorone starting material. Furthermore, the treatment described for isolating the diketone, which treatment consists of pouring the reaction mixture onto ice and then extracting the organic phase with ether, has proved laborious because of the presence of the acetic acid. It should furthermore be noted that the excess oxidizing agent is difficult to recover and that the handling and the use, for the oxidation, of solutions of chromium compounds, such as those mentioned above, in acetic acid/acetic anhydride mixtures, are operations which can present explosion hazards [see J. C. DAWBER, Chemistry and Industry, page 973 (1964)]. For these reasons, this process is difficult to adapt to an industrial scale.
Several processes for the preparation of 3,5,5- trimethyl-cyclohex-2-ene-1,4-dione by oxidation of .beta.-isophorone have also been described. French Pat. No. 1,446,246 has proposed oxidizing this ketone with oxygen, working in an alcoholic medium in the presence of a copper-II salt, of pyridine and of a tertiary amine; however, the method has a disadvantage that it only gives the diketone in a poor yield, of the order of 30%.
The French Patent Application published under No. 2,253,730 has described a process for the oxidation of .beta.-isophorone to 3,5,5-trimethyl-cyclohex-2-ene-1,4-dione by means of molecular oxygen or of a gas which contains molecular oxygen, such as air, in the presence of a catalyst consisting of a derivative (an oxide, salt or complex) of a transition metal, such as vanadium, chromium, copper, manganese, iron, cobalt or nickel, which derivative can be used by itself or deposited on the usual supports and especially on silica, charcoal, calcium or magnesium carbonate or bicarbonate, or diatomaceous earth. The best results are obtained in a homegeneous phase, that is to say where the catalyst used is a metal derivative which is soluble in the reaction mixture, such as the metal salts of carboxylic acids (acetates) or the acetylacetonates. However, even under these conditions, which are claimed to be the best, the yields remain mediocre because they do not exceed 55% relative to the .beta.-isophorone subjected to the oxidation, in spite of reaction times which are so long that this process loses any industrial value.
It is also possible to convert .beta.-isophorone to 3,5, 5-trimethyl-cyclohex-2-ene-1,4-dione indirectly, for example by treating the starting ketone with an organic peracid, then carrying out an alkaline hydrolysis of the resulting product, and thereafter oxidizing the 4-hydroxy-3,5,5-trimethyl-cyclohex-2-en-1-one formed in the preceding stage, by means of chromium trioxide. [see British Pat. No. 791,953 and J. N. MARX et al., Ietrahedron, 22, Suppl. 8, page 1 (1966)] The conversion of the .beta.-isophorone to the diketone in this manner entails the use of several stages and the utilization of numerous reactants, which limits the industrial value of the process proposed.
In the final analysis, none of the above-mentioned processes lend themselves to the industrial production of 3,5,5-trimethyl-cyclohex-2-ene-1,4-dione and industry is still looking for a process which makes it possible to convert .beta.-isophorone to this cyclohexenedione with, at one and the same time, good degress of conversion, good yields of diketone relative to the .beta.-isophorone converted, and relatively short reaction times.